Rimas VisGirda on tue 13 sep 11
Some time ago I read a posting that said if you see flames past the damper,=
=3D
you are wasting fuel... I'm not so sure about that... When I learned to (c=
=3D
/10 reduction) fire in updrafts, we were taught to push the damper in until=
=3D
there was a nice 5-6 inch blue/orange tongue of flame out the top spy hole=
=3D
and the bottom spy hole wasn't drawing any air; that condition always resu=
=3D
lted in flame out the damper. Correct me if I'm wrong, but that means there=
=3D
is not enough oxygen in the chamber to burn the fuel that is being put in =
=3D
therefore a reduction atmosphere in the chamber... Or to put it another way=
=3D
, all the oxygen is used up on the inside and there is unburned fuel floati=
=3D
ng around and being burned on the outside as it hits the air and spontaneou=
=3D
sly ignites because it's so damn hot....=3DA0 Conversely if there is NO fla=
me=3D
out the damper, that means all the fuel IS being burned inside the chamber=
=3D
and there may be free oxygen floating around, hence an oxydation
atmosphere in the chamber -or partial reduction? As an aside, back in my y=
=3D
outh, at the end of a firing we would push in the damper to get that sooty =
=3D
orange flame coming out of every hole and port for a few or 5 or 10 minutes=
=3D
... BUT that had nothing to do with the firing, it was just a visually cool=
=3D
thing to do... Didn't hurt the clay or glazes as far as I could tell... Up=
=3D
drafts were easy as the damper was at the top of the chamber. In my downdra=
=3D
fts I tried to put the damper as close to the chamber flue as was reasonabl=
=3D
e and used the same parameters for firing, some flame flicking past the dam=
=3D
per and licking out the top peep and no air being sucked into the bottom pe=
=3D
ep (which I used to put close to floor level). Wood kilns we fired we didn'=
=3D
t pay much attention to the stack, but to the hole at the top of the door. =
=3D
On the stoke there would be the orange flame gushing out, after a while the=
=3D
flame would start to "breathe" -get shorter and longer. The fire
masters job was to gauge what we called the Dragon's Breath and restoke be=
=3D
fore the flame got sucked completely back into the chamber... Again I think=
=3D
the same deal -flame out the hole, no oxygen inside hence reduction. No fl=
=3D
ame means excess oxygen in the chamber -and your copper reds go to Chun blu=
=3D
e... In Washington grad school, an engineer inspector came through the cera=
=3D
mics area when I was firing a kiln. He wanted to cite me on "wasting fuel" =
=3D
as there were flames present on the outside of the kiln. He indicated that =
=3D
a much more efficient firing is when there is no flame outside and all the =
=3D
fuel is consumed inside. I told him that that was OK for engineers and indu=
=3D
stries but that the results we were after were only possible with extra unb=
=3D
urned fuel in the chamber that had an affect on the clay and glazes of the =
=3D
resulting work... gave him a quick chemistry lesson about oxides and their =
=3D
properties. He went away happy. I may have been firing incorrectly
for all those years that I had gas kilns, but I don't think so. -Rimas=3D0=
A
Steve Mills on wed 14 sep 11
On the same agenda:
I was taught to do a reduction soak for about an hour at the beginning of t=
h=3D
at period. The reasoning for that was to build up a de-oxygenated core in t=
h=3D
e ware, so that during the long slow cooling period (in heavy, dense hard-b=
r=3D
ick Kilns), the tendency for the glazes to completely "re-oxadise"
was reduced.=3D20
I subsequently found that in my production Kiln (55 cu ft gross) which was =
a=3D
Fibre one I had to follow the same regime AND crash cool to 950oC to ensur=
e=3D
the depth of my Tenmoku glazes.=3D20
Old habits die hard as I still tend to soak at the start of reduction, and =
t=3D
hat's in a relatively small fibre kiln that cools quite quickly.=3D20
Steve M
Steve Mills
Bath
UK
www.mudslinger.me.uk
Sent from my Ipod touch
On 13 Sep 2011, at 22:45, Rimas VisGirda wrote:
> Some time ago I read a posting that said if you see flames past the dampe=
r=3D
, you are wasting fuel... I'm not so sure about that...=3D20
ivor and olive lewis on wed 14 sep 11
Rimas VisGirda gives us an adequate elementary overview of the firing
process which seems to explain what and why things happen as we adjust a
kiln's atmosphere. It might have had more universal application had he
discussed various types of fuel . Is this information equally applicable to
wood, gas, oil or any other carbonaceous material ( I have heard that in on=
e
developing nation discarded hospital dressings give excellent heat and in
another that shredded auto tyres are an ideal fuel).
One thing he has not told us is that some chemicals do not need to have a
fuel rich atmosphere to achieve reduction. Above a certain temperature they
spontaneously decompose and deliver free oxygen into the kiln atmosphere.
Regards,
Ivor Lewis,
REDHILL,
South Australia
.
Randall Moody on wed 14 sep 11
So just out of curiosity, when do you all reduce and for how long? Do you d=
o
two reductions such as a "body reduction" and a "glaze reduction" or just
one?
--
Randall in Atlanta
http://wrandallmoody.com
Lee on wed 14 sep 11
Blow holes determine reduction in traditional wood kilns.
--=3D20
--
Lee Love in Minneapolis
http://mingeisota.blogspot.com/
"Ta tIr na n-=3DF3g ar chul an tI=3D97tIr dlainn trina ch=3DE9ile"=3D97tha=
t is, "T=3D
he land
of eternal youth is behind the house, a beautiful land fluent within
itself." -- John O'Donohue
Ron Roy on wed 14 sep 11
Hi Rimas,
You are exactly right - to have reduction in the firing chamber you =3D20
have to have unburned fuel - when that unburned fuel combines with air =3D2=
0
coming in at the damper slot you get flame.
In fact it's one way to gauge if you are getting reduction and even how muc=
h=3D
.
If there is no flame at the damper then the atmosphere in the kiln is =3D20
oxidation or neutral (meaning just enough oxygen to burn all the fuel.
RR
Quoting Rimas VisGirda :
> Some time ago I read a posting that said if you see flames past the =3D20
> damper, you are wasting fuel... I'm not so sure about that... When I =3D2=
0
> learned to (c/10 reduction) fire in updrafts, we were taught to push =3D2=
0
> the damper in until there was a nice 5-6 inch blue/orange tongue of =3D20
> flame out the top spy hole and the bottom spy hole wasn't drawing =3D20
> any air; that condition always resulted in flame out the damper. =3D20
> Correct me if I'm wrong, but that means there is not enough oxygen =3D20
> in the chamber to burn the fuel that is being put in therefore a =3D20
> reduction atmosphere in the chamber... Or to put it another way, all =3D2=
0
> the oxygen is used up on the inside and there is unburned fuel =3D20
> floating around and being burned on the outside as it hits the air =3D20
> and spontaneously ignites because it's so damn hot....=3DA0 Conversely =
=3D20
> if there is NO flame out the damper, that means all the fuel IS =3D20
> being burned inside the chamber and there may be free oxygen =3D20
> floating around, hence an oxydation
> atmosphere in the chamber -or partial reduction? As an aside, back =3D20
> in my youth, at the end of a firing we would push in the damper to =3D20
> get that sooty orange flame coming out of every hole and port for a =3D20
> few or 5 or 10 minutes... BUT that had nothing to do with the =3D20
> firing, it was just a visually cool thing to do... Didn't hurt the =3D20
> clay or glazes as far as I could tell... Updrafts were easy as the =3D20
> damper was at the top of the chamber. In my downdrafts I tried to =3D20
> put the damper as close to the chamber flue as was reasonable and =3D20
> used the same parameters for firing, some flame flicking past the =3D20
> damper and licking out the top peep and no air being sucked into the =3D2=
0
> bottom peep (which I used to put close to floor level). Wood kilns =3D20
> we fired we didn't pay much attention to the stack, but to the hole =3D20
> at the top of the door. On the stoke there would be the orange flame =3D2=
0
> gushing out, after a while the flame would start to "breathe" -get =3D20
> shorter and longer. The fire
> masters job was to gauge what we called the Dragon's Breath and =3D20
> restoke before the flame got sucked completely back into the =3D20
> chamber... Again I think the same deal -flame out the hole, no =3D20
> oxygen inside hence reduction. No flame means excess oxygen in the =3D20
> chamber -and your copper reds go to Chun blue... In Washington grad =3D20
> school, an engineer inspector came through the ceramics area when I =3D20
> was firing a kiln. He wanted to cite me on "wasting fuel" as there =3D20
> were flames present on the outside of the kiln. He indicated that a =3D20
> much more efficient firing is when there is no flame outside and all =3D2=
0
> the fuel is consumed inside. I told him that that was OK for =3D20
> engineers and industries but that the results we were after were =3D20
> only possible with extra unburned fuel in the chamber that had an =3D20
> affect on the clay and glazes of the resulting work... gave him a =3D20
> quick chemistry lesson about oxides and their properties. He went =3D20
> away happy. I may have been firing incorrectly
> for all those years that I had gas kilns, but I don't think so. -Rimas
>
>
Vince Pitelka on wed 14 sep 11
Ron Roy wrote:
"You are exactly right - to have reduction in the firing chamber you have t=
o
have unburned fuel - when that unburned fuel combines with air coming in at
the damper slot you get flame. In fact it's one way to gauge if you are
getting reduction and even how much. If there is no flame at the damper the=
n
the atmosphere in the kiln is oxidation or neutral (meaning just enough
oxygen to burn all the fuel.
Hi Ron -
That's always been one of my favorite indications of reduction atmosphere,
because if you see flames at the damper slot, it's good evidence that there
is a reduction atmosphere throughout the kiln. In our downdraft kiln the
damper slab fits so snug that we do not have that indicator, and we can
gauge the kiln effectively by watching the spy holes, but in any kiln where
there is a little clearance in the damper slot this is always a good
indicator. And when a kiln is designed, placing a small spy hole on the
low-pressure side of the damper is a great idea.
Even when I can see flames beyond the damper, I rarely ever see flames at
the top of the stack. That seems like too much excess fuel.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
Vince Pitelka on thu 15 sep 11
Randall Moody wrote:
"So just out of curiosity, when do you all reduce and for how long? Do you
do two reductions such as a "body reduction" and a "glaze reduction" or jus=
t
one?
Hi Randall -
Way back in the 70s and early 80s I used to do a glaze reduction, but we ge=
t
far better results by doing a body-reduction and then maintaining a climbin=
g
reduction (partial reduction) up to maturation temperatures, with an
oxidation soak at the end to stop any bubbling and brighten the colors. Of
course much of this depends on the particular glazes in the firing and the
effects desired. For example, the body reduction traditionally serves to
bring out color and speckles in stoneware bodies, but we often use a fairl=
y
early body reduction (^012 or ^010) to encourage carbon trap shinos and
copper reds. With a climbing reduction (partial reduction) after body
reduction, the body-reduced effects are retained, whereas if you revert to
oxidation after body reduction you lose them. If the climbing reduction is
retained all the way to glaze maturation temperature, you generally get goo=
d
reduction effects without the muddiness that sometimes results from a
focused glaze reduction. I have always felt that a heavier glaze reduction
at glaze maturation just wastes fuel, but I'd love to hear opinions on this
from other potters.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net
http://iweb.tntech.edu/wpitelka/
Rimas VisGirda on thu 15 sep 11
My post regarding reduction was limited to gas fired kilns=3D
Hello Ivor,=3D0A=3D0AMy post regarding reduction was limited to gas fired k=
ilns=3D
. I have fired both propane and natural gas and there is no difference, wit=
=3D
h the exception of orifice size in the burner that I have observed. Hence t=
=3D
he conditions at the damper, top peep, and bottom peep remain the same rega=
=3D
rdless of type of gas, commonly propane and natural gas. I assume that the =
=3D
same would also be true of any gas one could get enough volume to use, buta=
=3D
ne, methane, etc. My fault for not stipulating that...=3D0A=3D0AI did respo=
nd i=3D
n an off-list post as to how I make adjustment using the factors I posted a=
=3D
bout and I paste it here:=3D0A=3D0ARegarding the bottom peep. I used to rol=
l a =3D
piece of newspaper, or you could =3D0Ause one of those really long wooden m=
at=3D
ches, light the tip of the paper =3D0Aand hold the flame at the (bottom) pe=
ep=3D
hole.=3D0A=3D0AIf the flame sucked in, push in the damper, repeat test=3D0=
AIf th=3D
e flame went out, or was blown away from the peep, pull out the damper, rep=
=3D
eat test=3D0AIf the flame just burned straight up, leave the damper alone.=
=3D0A=3D
=3D0ARegarding other fuels, I did talk about firing wood kilns in my post, =
th=3D
at is how I fire the wood kilns that I have had experience with, specifical=
=3D
ly a 6 chamber noborigama and single chamber kilns with an internal firebox=
=3D
. The first type of kiln that we had at H&V Pottery was wood and oil with e=
=3D
xternal fireboxes, we started with wood as it is easy to ignite at low temp=
=3D
and switched to oil at about red/low-orange heat as then the oil ignites e=
=3D
asily. That was over 40 years ago and I've forgotten the exact parameters t=
=3D
hat we used to gauge atmosphere... A cone 10-12 firing in that (hardbrick, =
=3D
approx 12 cuft) kiln was about 4-5 hours.=3DA0 My guess would be to keep th=
e =3D
fireboxes burning steadily until red heat say 2-3 hours, then turn on the o=
=3D
il. The oil in our case, I believe, approximated a gas fuel firing as the o=
=3D
il dripping onto an orange hot splash pan volitalizes into a gaseous state.=
=3D
Our biggest problem, I remember, was over-oiling the splash pan and
causing clinkers to grow between the bag wall and kiln wall and slowing do=
=3D
wn the firing. I have heard of people spraying the oil, as perhaps in a car=
=3D
buretor, but have no experience with that. Paul Soldner around the 70's had=
=3D
built some type of gadget to spray sawdust into a kiln as the fuel source =
=3D
with some success...=3D0A=3D0AAs you say, there are many fuels, shredded ti=
res,=3D
bandages, etc. that can be used but they are primarily in the third world =
=3D
(I'm assuming) and used for low temperature firing I can't imagine firing a=
=3D
medium sized kiln to cone 10 or 6 with bandages... But then there are many=
=3D
things that I couldn't imagine 20-30 years that I see around me on a daily=
=3D
basis...=3D0A=3D0AI am not aware of chemicals that "spontaneously decompos=
e to=3D
release free oxygen into the kiln atmosphere," but I'm not sure what that =
=3D
has to do with the price of tea in China, or Australia... My experience and=
=3D
discussion has been with basic ceramic materials and their oxides or carbo=
=3D
nates; iron, cobalt, copper, etc., I suppose I should have stipulated that.=
=3D
=3DA0 I could be wrong of course, but my (high fire) reduction atmosphere t=
he=3D
ory goes thus...=3D0A=3D0AIf the clay is an iron bearing stoneware, red iro=
n ox=3D
ide (rust) is the typical form of iron in the body, the lack of oxygen in a=
=3D
post red heat kiln strips the oxygen off of the red iron oxide and turns i=
=3D
t to black iron oxide. As the body (bisque) is still porous up to and somew=
=3D
hat beyond cone 04 (depending on the body) and the reducing atmosphere is i=
=3D
nitiated typically below cone 04, the iron throughout the body is converted=
=3D
to black iron oxide, turning the body into some shade of gray, or grey if =
=3D
you prefer, color. If the reduction is maintained until the glaze starts to=
=3D
form into its glasslike state, this seals the grey color under the glaze; =
=3D
and with a clear glaze the result is typically some shade of grey. At the c=
=3D
onclusion of the firing, excess oxygen enters the kiln, and re-oxidizes the=
=3D
surface of exposed bare clay thereby turning the black iron oxide back to =
=3D
red iron oxide at the surface. If you break a reduced stoneware piece open =
=3D
you will find
that the body is grey inside and just brown at the surface; and as I said =
=3D
earlier, the oxygen can't get to the body that is sealed under the molten g=
=3D
laze and so remains grey. Also if you were to pull a draw ring out of a fir=
=3D
ing reduction kiln and quickly quench it in water to not allow time for the=
=3D
skin to reoxidize you would find the draw ring is grey in color.=3D0A=3D0A=
Rega=3D
rding the colorants, in my experience oxides within a glaze, the reduction =
=3D
atmosphere has it's effect on the glaze chemistry before the glaze starts t=
=3D
o fuse because once the glaze is fused into a stable matrix it has become a=
=3D
n impervious liquid and is not affected by atmosphere with exception perhap=
=3D
s at the very surface. But glaze technology and what happens in and on a gl=
=3D
aze as it is forming is not my purview, so I won't venture there...=3D0A=3D=
0APr=3D
obably TMI and sure to be added to and/or corrected by those with more dire=
=3D
ct experience, experimentation, and knowledge than my memories of bygone te=
=3D
aching days can dredge up... I have heard that glaze technology is not "roc=
=3D
ket science" however, some of you might not know that I WAS a rocket scient=
=3D
ist in my youth and for me rocket science was much easier and preferable to=
=3D
glaze calculation...=3D0A=3D0ARegards, -Rimas
Rimas VisGirda on sat 17 sep 11
I'm pretty much in synch with Vince, maybe it's a West Coast thing... Back =
=3D
in the 60's in Sacramento we did a body reduction at 010 then partial reduc=
=3D
tion to the end and a "glaze reduction" at the end and closed the damper ov=
=3D
ernight. In our case we overdid the "glaze" reduction and did it just to se=
=3D
e the yellow flames and soot belching out of everywhere, more for performan=
=3D
ce than glaze chemistry... Through the years of teaching and various kilns =
=3D
my regimen wound up as follows. I started light reduction at around 012-015=
=3D
, never was much interested in shinos, but the early reduction was good for=
=3D
copper reds. Damper set so there's a bluish/orange 4-5 inch flame from the=
=3D
top peep, neutral (no pull or push) at the bottom peep. This resulted in a=
=3D
orange/blue flame=3DA0 (maybe 7-10 in) at the damper of a typical updraft.=
N=3D
o glaze reduction, just keep the same reduction til the end. Shut the burne=
=3D
rs, push in the damper for 5 minutes, open the damper wide and let
the kiln cool. I think closing the damper for a short time at the end serv=
=3D
ed as the oxidation soak that Vince was talking about. Pretty much the same=
=3D
for a downdraft at the peep holes except the top always seemed dirtier, bu=
=3D
t couldn't see the height of the flame past the damper. -Rimas=3D0A
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